The present invention relates to a recording apparatus which forms a picture element (pixel or PEL) by applying at least one or more pulses to a recording element disposed on a recording head. More particularly, the invention relates to a thermal head driving apparatus applied to a unit equiped with a thermal head, such as a printer, a facsimile, and a copying apparatus.
Generally, a sublimation type thermal printer comprises a thermal head having a recording unit which comprises a plurality of heating resistor elements arranged in a line. An image recording function such as printing or copying function is conducted by applying pulses to each of the heating resistor elements. In a multi-gradation-image recording apparatus, dot gradations are controlled by changing the length of the pulse applied to the heating resistor elements. The number of the heating resistor elements is the same as that of the dots (picture elements) which constitutes one picture element line. An image is formed on a record paper by selectively actuating the dots in the line while moving the record paper line by line.
In such a sublimation type thermal printer, heat value of each dot distributes unevenly in one picture element line due to unevenness of resistance value of each of the heat resistor elements.
In order to compensate for the unevenness of heat value of each resistor element, the heating resistor elements are classified in groups according to the resistance value of each element so that the time period for applying the pulse to the heating resistor elements is changed in accordance with the classified group. Such compensation is proposed in Japanese Patent Application Laying Open (Kokai) No. 59-45176.
Also, in general, a current applying and transferring type printer comprises a recording head comprising a recording unit having recording electrodes and common electrodes. The recording unit is urged against an ink sheet which comprises a resistance layer, a conductive layer and an ink layer of a melting type ink or a sublimation type ink. An electric current is applied to the ink sheet from the recording electrodes and the common electrodes so as to generate a Joule heat on the resistance layer of the ink sheet so that the ink is selectively transferred to the record paper.
Generally, the above-mentioned recording electrodes are formed on a ceramic substrate by a plating method which limits the electrode layer to approximately 70 .mu.m in thickness. The recording head is actually inclined with respect to the ink sheet when the head comes in contact with the ink sheet. Therefore, the width of the recording electrode actually in contact with the ink sheet is approximately 100 .mu.m. Appropriate thickness of the recording electrode is about 10 .mu.m to 30 .mu.m since accuracy of the positioning of the recording electrodes is degraded and the electrode layer becomes easy to be removed when the electrode thickness exceeds the above-mentioned appropriate range.
As a result, the area of the recording electrode coming in contact with the ink sheet can be relatively small, such as 140 .mu.m.times.30 .mu.m actually, although a desired area is 167 .mu.m.times.167 .mu.m with respect to the recording head having a picture element density of 6 dot/mm, for instance.
A way to widen the contacting area of the recording electrode for forming a picture element is to continuously move the recording electrodes and the ink sheet while applying current to the ink sheet from the recording electrodes and the common electrodes. Also, in the multi-gradation image recording system, the pulse width or the number of the pulses applied from the recording electrodes and the common electrodes to the ink sheet is changed in response to the density of the picture element.
In the multi-gradation printing operation using the above-mentioned sublimation type thermal printer, the dot gradation (density) is controlled by charging the width of the pulse applied to the heat resistor element. That is, when the dots are to be recorded in low density, a short pulse is applied to the heating resistors of the thermal head while moving the record paper, whereas when the dots are to be recorded in a high density, a long pulse is applied to the heating resistors. Therefore, the ending point of the high density dot is moved so that the center point thereof differs from that of the low density dot, which degrades the quality of the record image. Also, in the low density dot portion where white background is dominant, the image can have a rough appearance and high frequency effects may appear due to interaction from the high density dots, which can further degrades the quality of the recorded image.
When compensating for the uneven resistance value of the heating resistors by classifying them into a plurality of groups of a different resistance value, the time period for applying pulse is changed in accordance with the group. In accordance with this grouping compensation method, the pulse width of the pulse for the low resistance heating resistors is shorter than that for the high resistance heating resistors. Therefore, the dot size of the low resistance heating resistor element is smaller than that of the high resistance heating resistor element. Therefore, the white background portion becomes wide when the low resistance heating resistor elements are used to form dots. In order to even the dot density over the record paper, the pulses are applied to the heating resistor elements in such a way that the temperature of the low resistance heating resistor elements is higher than that of the high resistance heating resistor elements. Therefore, the service life of the low resistance heating resistor elements is shortened due to the high temperature operation thereof. Besides, uneven density portions are newly formed due to heat accumulated in the elements during the dot forming operation for the previous line.
Also, when a synthetic paper or an OHP paper can be used as the record paper, the paper is deformed and can lose luster from the surface thereof due to the high temperature of the low resistance heating resistor elements. Besides, a specific heat resistance treatment must be executed to the substrate of the ink sheet to cope with the high temperature from the low resistance heating resistor elements.
The ink color development is largely influenced by the heat diffusion time of the heat resistor elements especially in a high speed recording operation. Therefore, when the temperature of the low resistance heating resistors is raised in a short time, the ink may not be fully developed on the printing paper.
Also, in the multi-gradation image recording operation of the above-mentioned current applying and transferring type printer, the image could appear to be rough because the size of the picture element of the image is not even for each gradation and therefore the white background portion can be overemphasized in the low density portion of the recorded image.